In a semiconductor manufacturing apparatus, a machine tool, an industrial robot or the like, a positioning control is often carried out by using an electric motor. Precision in the positioning is greatly influenced by a resonance frequency of a machine. For this reason, it is desirable to grasp an accurate resonance frequency in advance and at the same time, to accurately carry out a measurement in a state in which a control system is incorporated and operated. From the necessity, there has conventionally been used a method of analyzing a frequency characteristic by using FFT (Fast Fourier Transform) to obtain a resonance frequency. The conventional art will be described with reference to the drawings.
FIG. 14 is a block diagram showing the structure of a conventional resonance frequency detecting device (for example, see Japanese Patent Unexamined Publication JP-A-6-78575) which is incorporated in an electric motor control system. In FIG. 14, the conventional resonance frequency detecting device has a command generator 11, controllers 21 and 22 for supplying a driving current to electric motors 31 and 32 upon receipt of a command signal C from the command generator 11, the electric motors 31 and 32, machines 41 and 42 to be driven by the electric motors 31 and 32, detectors 51 and 52 for detecting electric motor operating amounts m1 and m2 of the electric motors 31 and 32, FFT analyzers 121 and 122 for executing an FFT calculation over response signals S1 and S2 to be the outputs of the detectors 51 and 52 to calculate resonance frequency detection results f1 and f2, and output devices 81 and 82 for outputting the resonance frequency detection results f1 and f2.
In the conventional resonance frequency detecting device, the command generator 11 generates the command signal C and inputs the command signal C to the controllers 21 and 22. When the controllers 21 and 22 supply a current to the electric motors 31 and 32 in response to the command, the electric motors 31 and 32 drive the machines 41 and 42. At this time, the detectors 51 and 52 detect the electric motor operating amounts m1 and m2 such as the rotating positions or rotating speeds of the electric motors 31 and 32 and output the response signals S1 and S2. The FFT analyzers 121 and 122 execute the FFT calculation when inputting the response signals S1 and S2, and calculate the resonance frequency detection results f1 and f2. When inputting the resonance frequency detection results f1 and f2, the output devices 81 and 82 output numeric values or graphs which are visualized. Thus, a resonance frequency is measured every single shaft.
In the conventional art, however, the resonance frequency is measured every single shaft. For this reason, only a resonance frequency for one shaft to be operated in accordance with the command output from the command generator can be measured and the resonance frequencies of the other shafts to influence the operating shaft cannot be measured. Therefore, a servo regulation has conventionally been carried out every shaft. In some cases in which a plurality of shafts is to be operated, an oscillation is caused. For this reason, there is a problem in that it is hard to carry out the servo regulation.